JP7788414B2 - Optical isolator for the 780-820 nm wavelength band - Google Patents

Description

The present invention relates to an optical isolator for use in the 780-820 nm wavelength band.

Semiconductor lasers are the primary industrial lasers used in optical recording, medical, and optical measurement applications. Traditionally, compact semiconductor lasers with a wavelength of 780 nm have been used for optical recording on CDs, 800 nm pulsed semiconductor lasers have been used for bioimaging using two-photon fluorescence microscopy or second harmonic generation microscopy to visualize biomolecules, and semiconductor lasers in the infrared wavelength range of 780-800 nm have been used for laser length measuring instruments with positioning accuracy of a few nanometers to a few picometers.

A typical example of this type of semiconductor laser is the Fabry-Perot laser diode. The principle of laser oscillation is to use a resonator with two opposing reflective surfaces, formed by cleaving a crystal, at either end of the path through which light passes, and to emit a laser when stimulated emission becomes stronger than light absorption. These resonator structures are very simple, which has the advantage of allowing for the miniaturization of laser packages. However, when the emitted laser light is incident on the coupling end face of an optical fiber, for example, some of it may be reflected and become return light. If this return light is guided into the resonator, it interferes with the laser light that should be emitted, causing the problem of unstable laser output.

For stable operation of semiconductor lasers, it is therefore essential to place an optical isolator between the light source and the workpiece that transmits forward light and blocks backward light to prevent reflected light from returning to the light source, thereby blocking light that is reflected back from the optical fiber to the light source.

In order for this optical isolator to function, a Faraday rotation angle of approximately 45° is required. Specifically, light incident on an optical isolator has its polarization plane rotated 45° by the Faraday rotator, and is then transmitted through the angle-adjusted input and output polarizers. On the other hand, the non-reciprocity of the Faraday rotator is used to rotate the polarization plane of the returning light by 45° in the opposite direction, resulting in a polarization plane orthogonal to the input polarizer at 90°, and it cannot be transmitted. Optical isolators use this phenomenon to transmit light in only one direction, preventing reflected light from returning.

An optical isolator with this function consists of three main components: a Faraday rotator, a pair of polarizers arranged on the light input and output sides of the Faraday rotator, and a magnet that applies a magnetic field in the light transmission direction (optical axis direction) of the Faraday rotator. In this optical isolator, when light enters the Faraday rotator, a phenomenon occurs in which the plane of polarization rotates within the Faraday rotator. This phenomenon is generally called the Faraday effect, and the angle by which the plane of polarization rotates is called the Faraday rotation angle, and its magnitude θ is expressed by the following equation:
θ=V×H×L
Here, V is the Verdet constant, which is determined by the material of the Faraday rotator and the measurement wavelength, H is the magnetic flux density, and L is the length of the Faraday rotator (=sample length).

The factor that determines the size of an optical isolator is the size of the magnet that generates the magnetic flux density H. A larger magnet will have a larger magnetic flux density H, and using a smaller magnet to achieve miniaturization will reduce the magnetic flux density H. Therefore, for a Faraday rotator of the same length, it is necessary to develop a material with a larger Verdet constant.

TGG (terbium gallium garnet) (chemical formula: Tb ₃ Ga ₅ O ₁₂ ) has been known as a material for Faraday rotators used in the wavelength range of 780 nm to 820 nm. The Verdet constant of TGG in the 780 nm to 820 nm wavelength band is a small value of 0.25-0.29 min/(Oe·cm), and even the Verdet constant of TGG single crystals actually used is approximately 0.27 min/(Oe·cm) at a wavelength of 800 nm. With a Verdet constant of approximately 0.27 min/(Oe·cm), a larger magnet with an outer diameter of 3.5 cm and a length of 2.0 cm would be required to function as an optical isolator, which would result in a larger optical isolator, making it unsuitable as a Faraday rotator material. Note that 1 min represents 1/60 degrees.

Therefore, Patent Document 1 describes an oxide containing terbium oxide at a molar ratio of 50% or more as a material that can be used to miniaturize optical isolators. According to this description, when this oxide is used, it is possible to miniaturize optical isolators used at a wavelength of 800 nm when the Verdet constant V at that wavelength is 0.62 min/(Oe cm) or more and the magnetic flux density B (Oe) applied to the Faraday rotator is 0.5 × 10 ⁴ or less. Specifically, it is shown that with a molar ratio of 60% oxide, a Faraday rotation angle of 45° was obtained with a magnet having an inner diameter of 0.4 cm, an outer diameter of 3.5 cm, and a length of 2.0 cm, which had a magnetic flux density of 3950 [Oe] (= 0.395 [T]) at a sample length of 1.1 cm.

JP 2015-018145 A

However, in recent years, in the fields of optical recording, medical, and measurement applications using semiconductor lasers, the package sizes of semiconductor lasers used in the infrared wavelength band of 780-820 nm have been shrinking, and there is a demand for even smaller optical isolators installed outside these packages in the same wavelength band. The Faraday rotator described in Patent Document 1 has a Verdet constant of 0.62 min/(Oe·cm) at a wavelength of 800 nm, so the magnet size requires dimensions of an outer diameter of 3.5 cm and a length of 2.0 cm for a sample length of 1.1 cm. With these dimensions, the optical isolator would be larger than the laser package size, imposing layout restrictions on both. Therefore, a realistic outer diameter for a mountable optical isolator is 1.7 cm or less, less than half of 3.5 cm, and a length of 2.0 cm or less would be more suitable.

Therefore, conventional terbium oxide materials, with a Verdet constant of 0.62 min/(Oe·cm) at a wavelength of 800 nm, were not sufficient to meet this demand.

The present invention was developed in consideration of these circumstances, and aims to provide an even more compact optical isolator by combining a Faraday rotator with a large Faraday effect in the wavelength band of 780-820 nm with a magnet with a small outer diameter.

In order to solve the above problems, the present invention provides an optical isolator for the 780-820 nm wavelength band, which is made of a composite oxide containing an oxide represented by the following formula (A) as a main component, and which is also made of a Faraday rotator having a Verdet constant of 0.80 min/(Oe cm) or more at a wavelength of 800 nm, and a hollow magnet arranged on the outer periphery of the Faraday rotator:
The sample length L (cm) of the Faraday rotator is within the range of the following formula (1), and the magnetic flux density B (Oe) applied to the Faraday rotator is within the range of the following formula (2).
(Tb _x R _1-x ) ₂ O ₃ (A)
Here, in the above formula (A), x satisfies 0.8≦x≦1.0, and R contains at least one element selected from the group consisting of lanthanoid elements other than scandium, yttrium, and Tb (terbium).
0.8≦L≦1.1 (1)
B≦0.4×10 ⁴ (2)

In such an optical isolator, the Verdet constant is large, resulting in a strong Faraday effect, which requires a smaller magnetic field to achieve a given Faraday rotation angle, allowing the hollow magnet to be made smaller, thereby enabling the optical isolator to be made even more compact.

Furthermore, it is preferable that the oxide is a ceramic.

With such oxides, it is easy to realize a Faraday rotator with a large Verdet constant.

Furthermore, it is preferable that the Faraday rotator has an insertion loss of 1 dB or less and an extinction ratio of 30 dB or more over the sample length L (cm).

Such a Faraday rotator can make an optical isolator high performance with low loss and high isolation.

Furthermore, it is preferable that the hollow magnet be made of a neodymium-iron-boron (NdFeB) magnet.

With such a hollow magnet, it is possible to reliably apply magnetic flux density B to the Faraday rotator even in a small size.

As described above, the optical isolator for the 780-820 nm wavelength band of the present invention has a large Verdet constant, resulting in a high Faraday effect, requiring only a small magnetic field to obtain a predetermined Faraday rotation angle, and allowing the magnet to be made smaller, thereby achieving even greater miniaturization of the optical isolator.
Furthermore, the degree of freedom in the spatial dimensions within the laser device incorporating the optical isolator can be increased.
Furthermore, since the length can be made shorter than that of a conventional TGG Faraday rotator, this shortening of the length reduces absorption loss and insertion loss, which is an important characteristic of optical isolators.

1 is a cross-sectional view showing a configuration example of an optical isolator according to the present invention. FIG. 1 is a cross-sectional view schematically illustrating a configuration example of an optical isolator according to a first comparative example. FIG. 2 is a wavelength characteristic diagram of the transmittance of the Faraday rotator (without AR coating) used in Example 1 and Comparative Example 1. 1 is a diagram showing the relationship between the sample length L (0.8 to 1.1 cm) of a Faraday rotator and the magnitude of the magnetic flux density T (10 ⁴ Oe) at which the Faraday rotation angle is 45 degrees. FIG. 10 is a simulation diagram of magnetic flux density for calculating the magnet shape used in Example 1. FIG. 10 is a simulation diagram of magnetic flux density for calculating the magnet shape used in Comparative Example 1.

As mentioned above, there was a demand for even greater miniaturization of optical isolators for the 780-820 nm wavelength band.

After careful investigation, the inventors found that in the wavelength band of 780 nm to 820 nm, in order to keep the magnet outer diameter to φ1.2 cm or less and the length to 2.0 cm or less, the Verdet constant must be 0.80 min/(Oe·cm) or more; if the value is less than this, the magnitude of the specified magnetic field used will be 0.4×10 ⁴ (Oe) or more, making it difficult to further miniaturize the optical isolator.

To this end, the inventors conducted further research and developed an oxide material containing 80% or more terbium oxide in molar ratio, which has a Verdet constant of 0.85 min/(Oe·cm) or greater at a wavelength of 800 nm. They discovered that when this material is used in a Faraday rotator, its length can be shortened to 1.1 cm or less, making it possible to implement it within the above-mentioned magnet dimensions.

Furthermore, we discovered that combining the above-mentioned oxide material, which has a large Faraday effect, with a hollow magnet, which has a high magnetic flux density, would enable even greater miniaturization, leading to the present invention.

The following describes an embodiment of the present invention, but the present invention is not limited to this.

The optical isolator of the present invention is intended for use with laser light in the wavelength band of 780 to 820 nm. This laser includes semiconductor lasers. However, the optical isolator of the present invention can also be redesigned to be used with laser light in wavelength bands other than those mentioned above.

Figure 1 is a cross-sectional schematic diagram showing an example of the configuration of an optical isolator. In Figure 1, an input polarizer 1, a Faraday rotator 2, and an output polarizer 3 are arranged in sequence on an optical axis 6 extending from the input side on the left to the output side on the right. On the input side, the input polarizer 1 is fixed to a polarizer holder 4, and on the output side, the output polarizer 3 is also fixed to a polarizer holder 4. The polarization directions of the input and output light are adjusted and fixed to metal holders 5.

The shape of the Faraday rotator 2 is not particularly limited and may be a triangular prism or a square prism, but a cylindrical shape is preferable, so the following description will use a cylindrical Faraday rotator 2 as an example.

A hollow magnet 7 is arranged around the outer periphery of this Faraday rotator 2. If the Faraday rotator 2 is cylindrical, the hollow magnet 7 is preferably hollow and cylindrical, and the central axis of the Faraday rotator 2 and the central axis of the hollow portion of the hollow magnet 7 are preferably coaxial. Furthermore, the outer diameter of the Faraday rotator 2 and the inner diameter of the hollow portion of the hollow magnet 7 are preferably approximately the same, and alignment is preferably performed after the optical isolator is assembled. With this arrangement, the Faraday rotator 2 is positioned at the center of the hollow magnet 7.

Next, since the optical isolator of the present invention has a Faraday rotator with a Verdet constant of 0.80 min/(Oe·cm) or more at a wavelength of 800 nm, we will explain this Faraday rotator.

The Faraday rotator is made of an oxide material containing 80% or more terbium oxide, in terms of molar ratio, whose Verdet constant at a wavelength of 800 nm is 0.85 min/(Oe cm) or more. Specific examples of the material include an oxide represented by the following formula (A) as the main component, and may also contain a sintering aid.
(Tb _x R _1-x ) ₂ O ₃ (A)
In formula (A), x is 0.8≦x≦1.0, and R contains at least one element selected from the group consisting of lanthanoid elements other than scandium, yttrium, and Tb (terbium).

When using such oxide crystals as Faraday rotators in isolators, it is preferable to give the surface a mirror finish using an abrasive or the like after cutting. There are no particular restrictions on the abrasive, but colloidal silica is an example.

The Faraday rotator can be made of any material, as long as it is one of the above mentioned materials, but ceramics can be used. This is because ceramics can easily produce a Faraday rotator with a large Verdet constant. Hereinafter, this will be referred to as "transparent ceramics."

[Method for manufacturing transparent ceramics]
Transparent ceramics are produced using oxide powders in a desired blend ratio.
The method for producing transparent ceramics is not particularly limited and may be appropriately selected from conventionally known production methods. Examples of production methods for transparent ceramics include a hot isostatic pressing method, a method combining a solid-state method with a press molding method, and a vacuum sintering method using mold molding, and are described in, for example, Ikesue Akio, "From Optical Single Crystals to Optical Polycrystals," Applied Physics, Vol. 75, No. 5, pp. 579-583 (2006), Yanagiya Takahiro and Yagi Hideyoshi, "Current Status and Future of Ceramic Laser Materials," Laser Research, Vol. 36, No. 9, pp. 544-548 (2008), etc.

Below, we will explain an example of a method for producing transparent ceramics using hot isostatic pressing (HIP).

(Preparation of raw material powder)
The powder material of the oxide represented by formula (A) used as the starting material is preferably of high purity, preferably 99.9% by weight or more, more preferably 99.99% by weight or more, and even more preferably 99.999% by weight or _more . Note that the terbium oxide is not limited to _Tb2O3 , _{and Tb4O7} can also be used.

Transparent ceramics are manufactured using oxide powders in the desired blend ratio, but there are no particular restrictions on the shape or size of the powder particles used. Furthermore, there are no particular restrictions on the method for preparing these raw material powders. Raw material powders produced by dry milling, wet milling, coprecipitation, alkoxide hydrolysis, sol-gel, hydrothermal synthesis, spray pyrolysis, or any other synthesis method can be suitably used. In this case, it is sufficient that the oxide powder material is contained as the main component, and other components may also be intentionally contained as secondary components.

Here, "containing as the main component" means containing 50% by mass or more of the complex oxide represented by formula (A). The content of the complex oxide represented by formula (A) is preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 97% by mass or more, and particularly preferably 98% by mass or more.

Other commonly mentioned secondary components (components other than the main component) include dopants and fluxes that are added during single crystal growth, and sintering aids that are added during ceramic manufacturing.

Sintering aids include oxides of Group 4 elements such as titanium, zirconium, and hafnium, and oxides of Group 2 elements such as magnesium and calcium. Oxides of Group 4 elements and oxides of Group 2 elements may also be used simultaneously. These oxides have no absorption near wavelengths of 800 nm and are suitable for the transparent ceramics of the present invention. A specified amount of sintering aid may be added, preferably in an amount of more than 0.01 parts by mass and not more than 5.0 parts by mass, more preferably more than 0.03 parts by mass and not more than 3.0 parts by mass, and even more preferably more than 0.05 parts by mass and not more than 2.0 parts by mass, per 100 parts by mass of the composite oxide.

On the other hand, elements other than those mentioned above have problems such as absorption at wavelengths around 800 nm, difficulty in dissolving in rare earth oxides and therefore not reacting as sintering aids but instead precipitating on their own, excessive activity that prevents the crystal grain size from falling within the optimal range, and gradual reaction with moisture over a long period of time, causing the ceramic to become hygroscopic and devitrifying, making them unsuitable as sintering aids for producing the transparent ceramics of the present invention.

Next, the obtained raw powder material is weighed out to the desired molar ratio, and then at least one of a dispersant such as ammonium polyacrylate or ammonium polycarboxylate is added as needed to uniform the particle size, and a binder such as methyl cellulose, polyvinyl alcohol, or polyethylene glycol is added to obtain the desired molded product. A solvent such as water, methyl alcohol, or ethyl alcohol is then used to form a slurry, which is then subjected to conventional mixing and grinding using a ball mill or similar for several to 10-odd hours. The resulting slurry is then spray-dried to obtain dry spherical bodies.

Furthermore, the materials may be mixed and pulverized in a ball mill or the like and then calcined, or may be calcined and then further pulverized. Specifically, one example of a method is to mix and pulverize the materials in a ball mill or the like, then spray dry the resulting mixed powder, and then calcinate the resulting powder.

The firing temperature and firing time are not particularly limited, but a firing temperature of 600 to 1,600°C is preferred, 800 to 1,400°C is more preferred, and 800 to 1,100°C is even more preferred. The firing atmosphere is not particularly limited, and can be suitably selected from air, oxygen and oxygen-containing atmospheres, argon atmospheres, nitrogen gas atmospheres, and other inert atmospheres. The firing time is also not particularly limited and can be selected appropriately depending on the moisture content of the mixed powder and the firing temperature, but is preferably 1 to 20 hours, more preferably 5 to 15 hours. If firing is performed, it is preferable to further grind and mix the powder in a ball mill or the like for several to several tens of hours after firing.

Alternatively, to sharpen the average particle size distribution of the mixed powder and further increase its purity, the powder material may be dissolved, recrystallized, and pulverized before use as a raw material powder. Specifically, a high-purity (e.g., 99.9% by mass or higher) raw material powder is prepared and weighed to achieve a desired Tb ₂ O ₃ :R ₂ O ₃ molar ratio. The raw material powder is dissolved in a 1 mol/L aqueous nitric acid solution, mixed with a 1 mol/L aqueous ammonium sulfate solution, and then ultrapure water is added to adjust the concentration. While stirring the resulting solution, a 0.5 mol/L aqueous ammonium bicarbonate solution is added dropwise at a constant rate until the pH reaches 8. The solution is left at room temperature for several days while stirring, and then filtered, washed with ultrapure water, and dried at 150°C for several days.

The resulting mixed powder is placed in an alumina crucible and calcined in an inert atmosphere such as a nitrogen or argon atmosphere, preferably at 800 to 1,500°C, more preferably 1,000 to 1,400°C, and even more preferably 1,100 to 1,200°C, for preferably 0.5 to 10 hours, more preferably 1 to 7 hours, and even more preferably 2 to 4 hours. The inert atmosphere is used here to prevent changes in the valence of the terbium oxide.

[Manufacturing process]
The raw material powder is molded into a predetermined shape, degreased, sintered, and further processed by hot isostatic pressing (HIP) to produce a transparent complex oxide sintered body.

(molding)
In molding, a typical press molding process can be suitably used. That is, a very common press process in which the material is filled into a mold and pressurized from a certain direction, or a cold isostatic press (CIP) process in which the material is sealed in a deformable waterproof container and pressurized with hydrostatic pressure can be used. The applied pressure can be adjusted appropriately while checking the relative density of the resulting molded body, and is not particularly limited. In addition, other molding methods such as slip casting can also be suitably used in the molding process. In addition, the shape to be molded is not particularly limited and can be selected appropriately depending on the equipment used, etc., and an example is molding into a cylindrical shape.

(Degreasing)
A normal degreasing process can be suitably used for degreasing. That is, a temperature-raising degreasing process using a heating furnace can be performed. The temperature at this time is preferably 200 to 600°C, more preferably 250 to 400°C. The atmosphere in which degreasing is performed is not particularly limited, and air, oxygen, argon gas atmosphere, etc. can be suitably used.

(Sintering)
For sintering, a general sintering process can be suitably used. That is, a heat sintering process such as a resistance heating method or an induction heating method can be suitably used. The atmosphere is not particularly limited, and various atmospheres such as air, inert gas, oxygen gas, hydrogen gas, and helium gas can be suitably used, but sintering under reduced pressure (vacuum) can be more suitably used.

The degree of vacuum during sintering is preferably 1×10 ⁻¹ Pa or less, and more preferably 1×10 ⁻² Pa or less. Sintering conditions are preferably 1,300 to 1,800°C, more preferably 1,400 to 1,700°C, and even more preferably 1,400 to 1,600°C. The sintering time is preferably 1 to 50 hours, more preferably 2 to 25 hours, and even more preferably 5 to 20 hours. In this case, the temperature rise rate is preferably 100 to 500°C/hr, more preferably 200 to 400°C/hr, and even more preferably 250 to 350°C/hr up to about 1,200°C, and at temperatures higher than this, the temperature rise rate is preferably slowed down, and more preferably 20 to 100°C/hr.

(Processing by hot isostatic pressing (HIP))
After the sintering, a post-process such as hot isostatic pressing (HIP) is carried out to further increase transparency. In particular, in the case of a transparent complex oxide sintered body that does not exhibit translucency even after the sintering process is completed, it is preferable to proceed to HIP treatment, as this can eliminate bubbles in the sintered body and make it easier to obtain translucency.

The treatment temperature is preferably higher than the sintering temperature, preferably 1,300°C or higher. Furthermore, the treatment temperature in the HIP treatment is preferably 1,800°C or lower, more preferably 1,700°C or lower, because grain growth during the HIP treatment makes it difficult to remove bubbles. The treatment pressure is preferably 196 MPa or lower, which is convenient and can be used with commercially available HIP equipment. The treatment time is not particularly limited, but is preferably 50 hours or lower, more preferably 25 hours or lower, and even more preferably 10 hours or lower. Furthermore, it is preferably 15 minutes or longer, more preferably 30 minutes or longer, and even more preferably 1 hour or longer. The pressurized gas medium used here is preferably an inert gas such as argon, or Ar—O ₂ .

(processing)
When the obtained transparent body is used for optical purposes, both end faces on the optically used axis are optically polished to a length L (cm), and the flatness at this time is preferably λ/2 or less, and particularly preferably λ/8 or less, when λ = 633 nm. It is also possible to further reduce optical loss by appropriately forming an anti-reflection film on the optically polished surface.

In addition, in the present invention, the Verdet constant is not particularly limited as long as it is 0.80 min/(Oe·cm) or greater, but the upper limit is the Verdet constant associated with a 100% terbium oxide content. If the Verdet constant is less than 0.80 min/(Oe·cm), the magnet size required to achieve a Faraday rotation angle of 45° becomes large, making it difficult to miniaturize the optical isolator.

This Verdet constant can be measured according to standard methods, and is not particularly limited. Specifically, a piece of oxide of a specified thickness is cut out, mirror-polished, and attached to a permanent magnet with a known magnetic flux density as a Faraday rotator. The Verdet constant is then measured at a wavelength of 800 nm in air at a temperature of 25±10°C.

Table 1 shows specific composition examples of (Tb _x R _1-x ) ₂ O ₃ and the values of the Verdet constant at a wavelength of 800 nm.

From Table 1, for the three composition examples, when x = 0.8 and 1.0, the Verdet constant is 0.85 to 1.05 min/(Oe·cm), and all are above 0.80 min/(Oe·cm).

Furthermore, the length (sample length) L (cm) of the Faraday rotator of the present invention and the magnetic flux density B (Oe) of the mounted magnet satisfy the following formulas (1) and (2).
0.8≦L≦1.1 (1)
B≦0.4×10 ⁴ (2)
If the length of the Faraday rotator is less than 0.8 cm, the magnetic flux density required to obtain a Faraday rotation angle of 45 degrees exceeds 0.4 × 10 ⁴ Oe, which increases the external size of the magnet and makes miniaturization difficult. Furthermore, if the sample length of the Faraday rotator exceeds 1.1 cm, the applied magnetic flux density becomes extremely small, but conversely, the magnet becomes longer and the isolator length also becomes longer, making it difficult to further miniaturize the isolator.

The Faraday rotator of the present invention preferably has an insertion loss of 1 dB or less and an extinction ratio of 30 dB or more at a sample length L (cm) that satisfies the above formula. Furthermore, if the sample length L is within the range that satisfies the above formula, it is possible to fabricate an optical isolator with low loss and high isolation optical characteristics, which is preferable. Note that optical characteristics such as insertion loss and extinction ratio are measured in accordance with standard methods at a wavelength of 800 nm in air under measurement conditions of 25±10°C.

Furthermore, the Faraday rotator of the present invention preferably has a total light transmittance (total light transmittance) of 78% or more at a wavelength of 800 nm and a sample length L cm in the range of 0.8≦L≦1.1, and more preferably 80% or more. A higher total transmittance is preferable, but the upper limit is not particularly limited and is 100% or less.

The total light transmittance was measured using a UV-visible spectrophotometer V-670 manufactured by JASCO Corporation. Light was made uniform by multiple reflection in an integrating sphere with a highly reflective material applied to the inner wall, and light scattered by the turbidity of the sample was also measured based on the intensity of the captured light, and the total light transmittance was calculated based on the following formula. The wavelength in this case was 800 nm.
Total light transmittance=I/Io×100
In the above formula, I represents the transmitted light intensity (the intensity of light transmitted through a sample with a thickness of L cm), and Io represents the incident light intensity. Note that if the transmittance of the obtained oxide is not uniform and varies depending on the measurement point, the average transmittance of 10 arbitrary points is used as the transmittance of this oxide.

Next, we will explain the hollow magnet 7 used in the optical isolator of the present invention. It is preferable that this hollow magnet 7 be as small a permanent magnet as possible, and to obtain a large magnetic field strength, it is preferable to use a neodymium-iron-boron (NdFeB) magnet.

Generally, to achieve even higher magnetic flux density, a demagnetizing magnet configuration using multiple magnets, including a second hollow magnet 8 and a third hollow magnet 9, is required, as shown in Figure 2. In this case, a yoke structure made of carbon steel is required around the magnet, resulting in a larger external size.

Therefore, to miniaturize the optical isolator, it is preferable to use a single hollow magnet. As shown in Figure 1, the hollow magnet 7 according to the present invention preferably has its magnetic field polarity aligned with the optical axis 6, a polarizer material attached to the outer periphery of the hollow magnet 7, and an SUS metal holder 5 that can be welded and fixed. This configuration allows the magnet to be small and maximizes the magnetic flux density applied to the Faraday rotator 2.

In the basic design of the optical isolator of the present invention, reducing the outer diameter and length of the hollow magnet 7 is important for miniaturization. To achieve this, it is important to combine a Faraday rotator 2 with a large Faraday effect with a hollow magnet 7 material (magnet) with a high magnetic flux density in order to achieve even greater miniaturization.

Furthermore, optical damage to the Faraday rotator 2 caused by high-power light, which is a problem when using a semiconductor laser in an optical isolator, is determined by the transmittance of the Faraday rotator 2 and its length L. Therefore, to reduce this optical damage, it is advantageous for the Faraday rotator 2 to have a high transmittance and a short length L.

Furthermore, it is preferable that the optical isolator of the present invention has two or more polarization-dependent glass polarizers on the optical axis, and this configuration makes it possible to create a polarization-dependent optical isolator.

It is more preferable to provide two or more flat birefringent crystals and one or more 45-degree optical rotators on the optical axis 6, and this configuration also makes it possible to create a polarization-independent isolator. In this case, it is preferable that the optical axis of the flat birefringent crystal is oriented at approximately 45° with respect to the optical axis 6 and that the thickness is 1.0 cm or more. For example, when a rutile single crystal (TiO ₂ ) is used, it is possible to accommodate a beam diameter of φ1.0 mm, which is 1/10 of the thickness, and when an α-BBO crystal (BaB ₂ O ₄ ) is used, it is possible to accommodate a beam diameter of φ0.35 mm, which is about 1/30 of the thickness.

The above describes an embodiment of the present invention.

The present invention will be explained in detail below using examples and comparative examples, but the present invention is not limited to these.

Example 1
In Example 1, an optical isolator for the 780-820 nm wavelength band was fabricated with the configuration shown in Figure 1. For the input polarizer 1 and output polarizer 3, absorptive glass polarizers with high transparency/extinction ratio in the 780-820 nm wavelength band were used, and an anti-reflection film with a center wavelength of 800 nm was applied to their light-transmitting surfaces. In order to prevent reflected light from the light-transmitting surfaces from returning to the incident light path, the polarizers were inserted into a metal holder 5, with the bottom corners of each polarizer adhesively fixed onto a polarizer holder 4 tilted at an angle of 5 degrees.

Faraday rotator 2 was positioned at the center of the hollow portion of hollow magnet 7 and fixed in a position that maximized the magnetic field distribution generated by hollow magnet 7. The input polarizer 1 and output polarizer 3, arranged in the order of the incident light path, had their optical axes adjusted to maximize isolation characteristics in line with the 45-degree polarization angle rotated after passing through Faraday rotator 2, and the outer joints between polarizer holder 4 and metal holder 5 were fixed by laser welding. An anti-reflection film (not shown) with a center wavelength of 800 nm was applied to the light-transmitting surface of Faraday rotator 2.

Referring to Table 1, the material used for this Faraday rotator 2 was terbium/scandium oxide, which has a Verdet constant of 0.85 min/(Oe·cm) or greater at a wavelength of 800 nm. The sample length L of this Faraday rotator 2 was set to 1.1 cm, and a hollow magnet 7 made of a neodymium-iron-boron (NdFeB) magnet was placed around the periphery of the Faraday rotator 2, with a stainless steel metal housing placed around that.

To further explain the details of the Faraday rotator 2 of Example 1, the specific material used for the Faraday rotator 2 was a terbium/scandium oxide ceramic containing 80% terbium oxide in molar ratio terms. When the optical properties of this ceramic were measured at a wavelength of 800 nm, the insertion loss was 0.1 dB, the extinction ratio was 38 dB, and the Verdet constant was 0.85 min/(Oe·cm). The sample used for this measurement was cylindrical with an outer diameter of 2.3 cm and a length of 1.10 cm. Figure 3 shows the wavelength transmission characteristics of this Faraday rotator 2.

4 shows the relationship between the magnetic flux density Br (T=10 4 Oe) at which the Faraday rotation angle is 45 degrees and the sample length L (cm) when the sample length of the terbium/scandium oxide ceramic used in Example 1 is changed in 0.05 cm increments within the range of 0.80 to ^1.10 cm. Note that in FIGS. 3 and 4, the material described in Patent Document 1 is shown as a reference example.

In the case of the Faraday rotator 2 of Example 1, the sample length L is 1.10 cm, and the Verdet constant of the Faraday rotator 2 is 0.85 min/(Oe·cm). Therefore, when the magnetic flux density that results in a Faraday rotation angle of 45 degrees in this case is calculated from the relationship in Figure 4, it is found to be approximately 2900 [Oe] (= 0.29 [T]).

In the present invention, the lower limit of the Verdet constant is 0.80 min/(Oe·cm), and the material used for the Faraday rotator 2 is terbium/scandium, terbium/yttrium, or terbium/lutetium oxide, each containing at least 80% terbium oxide in terms of molar ratio. Since the lower limit of the sample length L is 0.8 cm, the upper limit of the magnetic flux density Br of this oxide, calculated from the relationship in Figure 4, is found to be 0.4 x 10 ⁴ [Oe] (= 0.4 [T]). Therefore, in order to further reduce the size of the optical isolator, it is preferable that the magnetic flux density B (Oe) of the hollow magnet used in the present invention satisfies the following formula (2):
B≦0.4×10 ⁴ (2)

Next, we will explain the shape of the hollow magnet that satisfies the magnetic flux density of Example 1. The shape of this hollow magnet was determined using the following magnetic field analysis. Specifically, the finite element method (JMAG-Designer) was selected as the magnetic field analysis method, a neodymium-iron-boron (NdFeB) magnet manufactured by Shin-Etsu Chemical Co., Ltd. was selected as the magnet material, and SUS stainless steel was selected as the material for the metal holder 5. Figure 5 also shows the simulation results of the magnetic flux density distribution obtained when the outer diameter of the hollow magnet is set to φ1.2 cm and the length (L') of the hollow magnet is used as a parameter.

The results in Figure 5 show that the hollow magnet shape showing the magnetic flux density distribution when the sample length L in Example 1 is 1.10 cm has an inner diameter of 0.3 cm, an outer diameter of 1.2 cm, and a length of 2.0 cm. Here, Z [cm] in the figure indicates the distance from the center on the optical axis 6, and the sample length L [cm] is calculated as 2 x Z. In other words, the sample length L in Figure 5 is 1.10 cm when the horizontal axis Z is 0.55 cm.

When an optical isolator was assembled at a wavelength of 800 nm, an optical isolator with optical characteristics of an insertion loss of 0.5 dB and an isolation of 38 dB was successfully produced.

<Comparative Example 1>
In Comparative Example 1, an optical isolator was fabricated using a TGG single crystal (Verdet constant 0.27 min/(Oe cm)) as the Faraday rotator. The magnetic flux density to be applied to this TGG single crystal was calculated for sample lengths ranging from 0.80 to 1.10 cm, resulting in the results shown in FIG. 4. In particular, it can be seen that the magnetic flux density required for Comparative Example 1 at a sample length of 1.1 cm, the same as in Example 1, is approximately 9,100 [Oe] (=0.91 [T]). Therefore, in order to calculate the hollow magnet shape in the same way as in Example 1, the outer diameter was set to φ2.5 cm, and the simulation results of the magnetic flux density distribution obtained using the length (MT) of the hollow magnet 7 shown in FIG. 2 as a parameter are shown in FIG. 6.

The results in Figure 6 show that the hollow magnet shape that satisfies the magnetic flux density requirements of Comparative Example 1 is a demagnetizing field configuration consisting of hollow magnet 7, second hollow magnet 8, and third hollow magnet 9, with an inner diameter of 0.3 cm, an outer diameter of 2.5 cm, and a total length of 6.5 cm, including the length MT of hollow magnet 7, which is 2.0 cm. Here, Z [cm] in the figure indicates the distance from the center of the optical axis 6, and the sample length L [cm] is calculated as 2 x Z.

Comparing the above results between Example 1 and Comparative Example 1, it was confirmed that Example 1 could achieve a size reduction of more than 90% in terms of volume ratio calculated from the outer diameter and length of the magnets of both examples compared to the TGG optical isolator of Comparative Example 1.

Furthermore, since the Verdet constant is wavelength dependent and is generally known to decrease at longer wavelengths, the Verdet constant was also evaluated at 820 nm, the upper wavelength limit in the 780-820 nm band. The TGG of Comparative Example 1 was found to have a value of 0.26 min/(Oe·cm), while the TGG of Example 1 was found to have a value of 0.80 min/(Oe·cm), confirming that even at the long wavelength of 800 nm, it has more than three times the performance.

Therefore, it was confirmed that the optical isolator of this embodiment can be put into practical use as a sufficiently compact optical isolator that exhibits low loss and high isolation characteristics in the 780-820 nm band.

The present invention includes the following aspects.
[1]:
An optical isolator comprising a composite oxide containing an oxide represented by the following formula (A) as a main component, a Faraday rotator having a Verdet constant of 0.80 min/(Oe cm) or more at a wavelength of 800 nm, and a hollow magnet disposed on the outer periphery of the Faraday rotator:
An optical isolator for the 780-820 nm wavelength band, characterized in that the sample length L (cm) of the Faraday rotator is within the range of the following formula (1), and the magnetic flux density B (Oe) applied to the Faraday rotator is within the range of the following formula (2).
(Tb _x R _1-x ) ₂ O ₃ (A)
Here, in the above formula (A), x satisfies 0.8≦x≦1.0, and R contains at least one element selected from the group consisting of lanthanoid elements other than scandium, yttrium, and Tb (terbium).
0.8≦L≦1.1 (1)
B≦0.4×10 ⁴ (2)
[2]:
The optical isolator according to the above [1], wherein the oxide is a ceramic.
[3]:
The optical isolator according to [1] or [2], wherein the Faraday rotator has an insertion loss of 1 dB or less and an extinction ratio of 30 dB or more in the sample length L (cm).
[4]:
The optical isolator according to any one of [1] to [3] above, wherein the hollow magnet is made of a neodymium-iron-boron (NdFeB) magnet.

The present invention is not limited to the above-described embodiments. The above-described embodiments are merely examples, and any configuration that is substantially identical to the technical concept described in the claims of the present invention and that provides similar effects is within the technical scope of the present invention.

1...Input polarizer, 2...Faraday rotator, 3...Output polarizer, 4...Polarizer holder, 5...Metal holder, 6...Optical axis, 7...Hollow magnet, 8...Second hollow magnet, 9...Third hollow magnet.

Claims (4)

An optical isolator comprising a composite oxide containing an oxide represented by the following formula (A) as a main component, a Faraday rotator having a Verdet constant of 0.80 min/(Oe cm) or more at a wavelength of 800 nm, and a hollow magnet disposed on the outer periphery of the Faraday rotator:
An optical isolator for the 780-820 nm wavelength band, characterized in that the sample length L (cm) of the Faraday rotator is within the range of the following formula (1), and the magnetic flux density B (Oe) applied to the Faraday rotator is within the range of the following formula (2).
(Tb _x R _1-x ) ₂ O ₃ (A)
Here, in the above formula (A), x satisfies 0.8≦x≦1.0, and R contains at least one element selected from the group consisting of lanthanoid elements other than scandium, yttrium, and Tb (terbium).
0.8≦L≦1.1 (1)
B≦0.4×10 ⁴ (2) An optical isolator as described in claim 1, characterized in that the oxide is a ceramic. The optical isolator described in claim 1, characterized in that the Faraday rotator has an insertion loss of 1 dB or less and an extinction ratio of 30 dB or more over the sample length L (cm). An optical isolator as described in any one of claims 1 to 3, characterized in that the hollow magnet is made of a neodymium-iron-boron (NdFeB) magnet.